Noise reduction apparatus of frame recursive type

Abstract

Disclosed is a noise reduction apparatus of a frame recursive type which can reduce occurrence of an afterimage phenomenon such as blurring or trailing of contours of motion images even when the difference between image frames is small. In the noise reduction apparatus, a frame recursive type filter generates a differential image signal representing a difference between a current input image signal and a delayed image signal obtained by delaying the input image signal by at least one frame period, reduces noise in the differential image signal thereby to generate a feedback image signal, and adds the feedback image signal to the current input image signal. A level adjuster adjusts a level of the feedback image signal to a lower level as a level of a luminance or color component of the input image signal is decreased.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a technique for reducing noise in an image signal. [0003] 2. Description of the Related Art [0004] In motion images, there is usually strong correlation between image frames that are sequentially ordered in time. There is further a known fact that correlation is very low between noise components contained in the image frames that are sequentially ordered in time. As a technique for reducing the noise based on that known fact, a noise reduction technique of a frame recursive type is known. [0005]FIG. 1 is a diagram showing a schematic configuration of a conventional noise reduction apparatus 100 of the frame recursive type. In this noise reduction apparatus 100, an input image signal is a digital image signal such as a luminance signal (Y signal), color-difference signals (Cb signal and Cr signal), primary color signals (R signal, G signal and B signal) or the like. A multiplier 101 m...

AI Technical Summary

Benefits of technology

[0010] In view of the foregoing, it is an object of the present invention to provide a noise reduction apparatus of the frame recursive type which can reduce occurrence of the afterimage phenomenon such as blurring or trailing of the contours of motion images even when the difference between the image frames is small.

[0011] According to a first aspect of the present invention, there is provided a noise reduction apparatus of a frame recursive type which reduces noise in an input image signal. The noise reduction apparatus comprises: a frame recursive type filter for generating a differential image signal representing a difference between a current input image signal and a delayed image signal that is obtained by delaying the input image signal by at least one frame period, reducing noise in the differential image signal thereby to generate a feedback image signal, and adding the feedback image signal to the current input image signal; and a level adjuster for adjusting a level of the feedback image signal to a lower level as a level of a luminance or color component of the input image signal is decreased.

[0012] According to a second aspect of the present invention, there is provided a noise reduction apparatus of a frame recursive type which reduces noise in an input image signal. The noise reduction apparatus comprises: a frame recursive type filter for generating a differential image signal representing a difference between a current input image signal and a delayed image signal that is obtained by delaying the input image signal by at least one frame period, reducing noise in the differential image signal thereby to generate a feedback image signal, and adding the feedback image signal to the current input image signal; a frequency analyzer for measuring a spatial frequency distribution of the current input image signal; and a level adjuster for adjusting a level of the feedback image signal to a lower level as a ratio of a low frequency band component to the current input image signal is increased, on the basis of the measured spatial frequency distribution.

[0013] According to a third aspect of the present invention, there is provided a noise reduction apparatus of a frame recursive type which reduces noise in an input image signal. The noise reduction apparatus comprises: a frame recursive type filter for generating a differential image signal representing a difference between a current input image signal and a delayed image signal that is obtained by delaying the input image signal by at least one frame period, reducing noise in the differential image signal thereby to generate a feedback image signal, and adding the feedback image signal to the current input image signal; a hue analyzer for measuring a hue angle on the basis of a color difference component of the input image signal; and a level adjuster for adjusting a level of the feedback image signal to a lower level as the measured hue angle is nearer to a predetermined angle.

Problems solved by technology

However, when the effect of reducing the noise is enhanced, an afterimage phenomenon such as blurring or trailing of the contour of the motion image occurs and lowers image quality.

Particularly, when the difference between the image frames is small, the motion detector 105 can fail in detection of the motion amount.

In this case, the recursive factor setting unit 106 cannot set a recursive factor with respect to the delayed image signal to a proper value, thus causing blurring or trailing of the contour of the motion image to occur.

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